The present invention relates to a method for measuring residual stress in nickel-base alloys by x-ray diffraction.
X-ray diffraction is used to determine residual strain by employing the atomic lattice as a strain gage. The spacing of a particular set of atomic planes in this lattice is measured by observing the angles at which diffracted monochromatic x-rays are detected. In typical techniques with macroscopic strain gages, it is possible to determine the difference in extension between the loaded and unloaded conditions. In x-ray diffraction, however, it is not appropriate to determine the difference between a separate, unstrained sample of a metal, e.g. an annealed powder, and the metal that forms part of a work piece of interest. This is due to the fact that the interatomic spacing is very sensitive to small chemical differences, such as those that would occur between the two samples, and the resulting differences in spacing would be far greater than those brought about by the strain.
Accordingly, it is the typical practice to use the same sample of metal, but in two different orientations, for the two necessary measurements. Typically, this sample is a small portion of metal at some location in the surface of a work piece believed to be subject to residual elastic strain. If, within the same small incremental volume of metal, the spacing varies with direction, this is an indication of the presence of strain.
A first measurement is taken of the spacing of planes normal to the sample surface. This measurement is made at the sample position used for routine diffraction work, and the position represents the geometry providing optimum resolution and accuracy. The process differs from routine diffraction work primarily in that only a single peak, at a very high diffraction angle, is examined. The positions of such peaks are very sensitive to small changes in spacing. For the second measurement, it is necessary to turn the sample such that the surface no longer is in the plane specified by this optimum geometry.
This procedure introduces various errors external to the sample, resulting in observable shifts in diffractogram peaks and a loss of peak intensity and resolution. In an attempt to eliminate these errors, mathematical corrections based on empirical models are applied to the results.
Another source of error in the measurement may arise from the use of curved sample surfaces, such as in U-bends. The strain condition of such pieces is often specifically of interest, or chosen for testing purposes. In these cases, only a small part of the surface is in the proper plane for accurate x-ray diffraction measurement; any other portion of the surface that contributes to the signal detracts from the accuracy, the resolution, and the validity of the mathematical corrections.
It is therefore desirable to incorporate into this method of diffraction determination an additional material which when measured is comparable to the sample, so as to create an internal standard. This standard material would be known to be unstrained. Thus, any shift in the peak diffractogram position caused by rotating the standard material and the sample could be attributed to errors external to the sample, since precisely the same errors would apply to both the additional material and the sample. Further, such a standard material would desirably be available in very pure form and have exceptionally sharp diffractogram peaks subject to accurate determination. It is also desirable that the diffractogram peak of the standard material be close to the peak of the sample for which a measurement is desired, and yet not so close as to interfere with the sample's peak. In such a case, the measurement would take the form of observing the changes in the differences in position between the sample and standard material peaks.
It is therefore also desirable to use only a minimum portion of the surface which can produce a satisfactory diffraction signal so as to minimize variability of results and to eliminate or mask off the unsatisfactory signals caused by other portions of the surface with a material that is relatively opaque to x-rays.